Machines are available for cutting hard surface such as concrete and asphalt. To cut such hard surfaces, a wheel is rotated about its axis and cutting tools mounted on the wheel are applied against the surface and each tool removes a small portion of hardened material thereby advancing the cut.
To maximize the useful life of such cutting tools, the tools are rotatably mounted about a longitudinal axis and have a cylindrical mounting portion rotatably fitted into a cylindrical aperture on a mounting block. Force is applied from the mounting block on the wheel against a rearward surface of an annular flange on the tool which rests upon a forward surface of the mounting block.
The body of the tool to which the tungsten carbide cutting tip is attached and the tool mounting block into which the cylindrical mounting portion of the tool is fitted are made of cold formed or forged steel which is much softer than the tungsten carbide cutting tip. As the machine cuts hard surfaces such as asphalt or concrete, fragments of the broken surface are forced across the tapered forward portion of the tool and around the forward and side portions of the mounting block causing wear or wash away of the material which makes up both the tool body and the mounting block. After a substantial portion of the forward end of the tool has been worn away, the tool must be replaced. Similarly, after a substantial portion of the body of the mounting block has been washed away, the tool mounting block must also be replaced.
The rotation of the tool within the block occurs as a result of an uneven application of forces against the tool as it is applied to the hardened surface and, therefore, the mated annular surfaces on the block and on the tool, which transfer force from the block to the tool, also serves as a bearing surface for the rotation of the tool within the block. Over a period of time, particles of hardened material broken up by the tool work along the forward surface of the mounting block and under the rearward surface of the flange causing the mated surfaces to become irregular and thereby increasing the friction between the surfaces. The increased friction reduces the rotatability of the tool within the block. A tool which does not rotate within the mounting block will wear unevenly, thereby substantially reducing its useful life.
In recent years, the annular flanges behind the forward cutting ends of tools have been made larger in diameter to provide protection to the mounting block such that the portion of the body of the mounting block behind the flange will remain intact much longer than the body of the tool retained therein. As many as one hundred tools or more may be worn out before a mounting block suffers such wear that it must be replaced.
Although the presence of the enlarged flange on such tools protects portions of the body of such mounting blocks against wash away, particles of hardened material nonetheless work their way between the abutting surfaces of the mounting block and the tool and cause the forward surfaces of the mounting block and the inner surface of the cylindrical aperture extending through the mounting block to become worn. As a result of the wear on these two surfaces, a replacement tool inserted in the mounting block will not be snugly retained in the aperture, nor will the replacement tool rotate freely therein. When a replacement tool is inserted into a mounting block having a worn bore, the replacement tool will have a useful life which is much shorter than that of the original tool.
The flange of the tool which protects the mounting block from wash away also causes wear to the forward surface of the mounting block. After a number of tools have become worn out in a mounting block, the friction between the rear surface of the flange and the forward surface of the mounting block will cause a counterbore to be worn in the forward surface of the mounting block. When a new replacement tool is inserted into a mounting block which already has a counterbore worn by the flanges of prior tools, the flange of the replacement tool can bind against the inner circumference of the wall of the counterbore and prevent rotation of the replacement tool, which will lead to the premature failure of the tool.
It would be desirable, therefore, to provide a mounting block for which the critical surfaces which permits a tool to rotate in the bore will be resistant to wear to thereby further extend the useful life of the mounting block.
Efforts have been made to provide a tungsten carbide wear ring at the forward end of a mounting block as shown by Mills, U.S. Pat. No. 4,932,723. Efforts have also been made to protect the bore of a mounting block against excessive wear as shown by Kniff, U.S. Pat. No. 3,512,838. These efforts, however, have been less than successful for a number of reasons. First, tungsten carbide, which is the most desirable material for use in such inserts, is extremely brittle much like glass and easily fractures. Fracturing can occur for any of a number of reasons, one of which is expansion and contraction. The tools and mounting blocks of a cutting machine become extremely hot while in use (up to 600xc2x0 F.) and the parts are continuously sprayed with water to prevent over heating and to suppress dust. As a result, the tools and mounting blocks are alternately heated as the tool cuts into hard material and cooled as the wheel rotates around from the end of one cut to the beginning of the next. The coefficient of expansion for tungsten carbide (0.00000239 per unit length/xc2x0 F.) is approximately one third that of the coefficient of expansion for cast or wrought iron (0.00000661 per unit length/xc2x0 F.), and the alternate heating and cooling of the brazed parts causes internal stresses within the tungsten carbide. The internal stresses can cause microscopic fractures to occur within the tungsten carbide and the microscopic fractures will lead to the rapid deterioration of the part. To prevent such microscopic deterioration, a tungsten carbide wear ring should have a minimum thickness of at least xe2x85x9 inch and should be encased in braze material so that only the contact wear surface is exposed.
It is not practical to make a tungsten carbide part having both a cylindrical portion which would fit within a bore of a tool and a wear ring flange because internal stresses would always lead to failure of the part at the junction between the cylindrical portion and the flange portion.
Another problem which has lead to the failure of prior tungsten carbide inserts arises from the difficulty of brazing the parts together. Irregularly shaped parts such as those having both a cylindrical portion and a flange portion do not retain liquefied braze material between the parts during the brazing and as a result, portions thereof, such as the flange, will fracture off the mounting block because it is not adequately retained by braze material. If the wear ring is not encased in metal the tungsten carbide will be gradually chipped away as a result of impacts with pieces of hard material loosened by the tool as it cuts, thereby shortening the life of the mounting block the ring was intended to protect.
Another problem with a tungsten carbide wear ring is caused by wash away. The flange of the tool bodies protects a portion of the mounting block from wash away, but the portions of the block which extend beyond the outer diameter of the flange are still washed away over time. Since the wear ring must have a diameter approximately equal to the diameter of the flange, the metal encasing the outer circumference of the wear ring, which is unprotected by the flange, will be gradually washed away leaving the ring exposed and subject to being chipped away as described above.
It would be desirable to provide an improved insert which could protect the surfaces of a mounting block from becoming prematurely worn but would not be subject to fracturing. It would also be desirable to provide an insert which would more readily retain brazing material between the parts during the brazing operation.
Briefly, the present invention is embodied in a tool mounting block having a forward surface and an attachment portion whereby the mounting block is attachable to a cutting machine. The tool block has a forward surface and an aperture extending through the body of the block and an opening in the forward surface. A countersink is provided in the forward surface around the aperture and an annular insert is fitted within the countersink.
The annular insert has a central opening axially aligned with the axis of the cylindrical aperture in the mounting block and has a planar forward surface which is forwardly offset with respect to the forward surface of the mounting block. The insert further has a frustoconical outer wall and a planar rearward surface which is parallel to the forward surface. In the preferred embodiment, the annular insert is made of tungsten carbide and is bonded into the countersink in the mounting block with a suitable bonding material such as a braze.
A tool having a generally tapered body with a forward cutting end and a tungsten carbide tip at the forward end thereof and has an annular flange positioned rearward of the tapered body and a cylindrical mounting portion axially aligned behind the forward cutting end of the flange. The cylindrical mounting portion of the tool is rotatably fitted into the cylindrical aperture of the mounting block to permit rotation of the tool. Since the tungsten carbide insert is forwardly offset a short distance above the forward surface of the block, the rearward surface of the bit rotates on the forward surface of the insert and does not cause wear to the metal of the block body.
The tungsten carbide of the annular insert is much harder than the steel from which the body of the tool is made and, therefore, the steel of the tool becomes worn away by particles of hard material which work their way between the abutting surfaces of the annular insert and the tool while the surfaces of the tungsten carbide insert suffer very little wear. Typically, the steel of the tool wears away approximately ten times faster than the tungsten carbide of the insert is worn away.
After a tool mounted in such a block has become worn, the tool can be removed and the forward surface of the tungsten carbide annular insert will not be gouged or damaged so as to cause a substantial increase in the resistance to rotation when a new tool is inserted into the block. Similarly, the cylindrical inner surface of the bore of the annular insert will not have become worn away as a result of particles of hardened material working their way between the parts and when the cylindrical mounting portion of a replacement tool is inserted there, it will be snugly retained therein. In addition to the above, the coefficient of friction between two surfaces where one is steel and one is tungsten carbide is less than the coefficient of friction between two surfaces where both are steel. As a result, the useful life of the mounting block is extended and it can be expected to not require replacing until well over a hundred tools have become worn out.
In another embodiment of the invention, the aperture into which the tool is received extends to a rear surface of the mounting block and a counter bore is provided in the rear surface around the aperture. A second annular insert of tungsten carbide is provided in the counter bore in the rear surface such that a tungsten carbide ring is provided around both the forward and rearward ends of the aperture to ensure that both ends thereof are wear resistant.